Georeferencing Images from Google Earth USING ARCGIS Google Earth has great high resolution aerial imagery available free to the public. It's a great GIS viewer program and it's easy to use, however sometimes I need to do more with the imagery so I import it into ArcGIS by georeferencing it. This tutorial will explain an easy and accurate way to georeference images from Google Earth in ArcMap. I will be using 2000 TIGER/Line data in Little Rock, AR for demonstration purposes. I start off with a base map of the area of interest in ArcMap using the TIGER data. This is not a neccessary step, I just want to show how the image will line up properly to the data set when it is finalized. Recent Entries Rails 2.3.5 on Macbook Pro, MySQL Gem Issues(02/17 /2010) Danielle Davis Art/Photography Site Launch(08/31/2009) ASP.Net Session Timeout Resolved(02/03/2010) Niagara Falls(08/24/2009) Grand Lake Stream, Maine Trip(12/27/2009) Paying My Bills Conversations With Myself http://conversationswithmyself.com/241 1 of 17 10/23/2010 7:43 PM
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Georeferencing Images from Google EarthUSING ARCGIS
Google Earth has great high resolution aerial imagery available freeto the public. It's a great GIS viewer program and it's easy to use,however sometimes I need to do more with the imagery so I importit into ArcGIS by georeferencing it.
This tutorial will explain an easy and accurate way to georeferenceimages from Google Earth in ArcMap. I will be using 2000TIGER/Line data in Little Rock, AR for demonstration purposes.
I start off with a base map of the area of interest in ArcMap using theTIGER data. This is not a neccessary step, I just want to show howthe image will line up properly to the data set when it is finalized.
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Next open Google Earth and zoom in to the area of interest. I usuallyzoom out one magnitude more than my area of interest in ArcGIS soI will have enough aerial image to cover the area.
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Now I will pick out 4 locations on the Google Earth map that areequally distributed and are good visual landmarks. This will allow usto easly find them in ArcMap when we get the image imported. Try touse features that are permanent (i.e. buildings, intersections,bridges) and are easy to spot in the image.
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We need to get the coordinates for each of these points while inGoogle Earth. Right-click your point and choose "Edit" to pull up theproperties of the point. Check the "Advanced" checkbox and click onthe "Location" tab to view the latitude and longitude.
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5 of 17 10/23/2010 7:43 PM
station xCoord yCoord
4 -92.263082 34.742178
2 -92.282934 34.741464
3 -92.283584 34.752602
1 -92.265551 34.754582
Save the file as pointLocations.txt, we will be using it later in ArcGIS.
Next turn all the points off by unchecking them in the Places list andclick "File > Save Image". Save the image in an accessable place forArcGIS.
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In ArcGIS make sure you have the Georeferencing toolbar open. Ifnot turn it on by clicking "View > Toolbars > Georeferencing". Nextadd the exported image to your view via the Add Data button.Browse to your exported aerial image and add it to your layers. Youwill see a prompt regarding missing geospatial information, justignore it. You will not be able to see the image immediately on themap, it will only appear in the layers list.
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7 of 17 10/23/2010 7:43 PM
Click on the Georeferencing toolbar and select "Fit to Display" Theimage will now appear in your view. If you can't see the image, itmay be because it is under other layers in the map. I usually turn offany polygon data that may interfere with my georeferencing. In thiscase I will turn of Urban Areas and Water polygons.
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8 of 17 10/23/2010 7:43 PM
Now we will add the text file we created earlier as a data layer. Clickthe Add Data button and browse to your pointLocations.txt file. It willappear in the Source tab of the layers window.
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9 of 17 10/23/2010 7:43 PM
Right-Click the text file in the source list and select "Display XYData". Match up the X and Y fields respectively and click OK.
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10 of 17 10/23/2010 7:43 PM
This creates a new data layer called pointLocations.txt Events andinlcudes the 4 points on the map now. Right-Click the layer andselect Zoom to Layer to view the entire points. I usually change thecolor of the symbol to something bright so I can see it clearly.
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11 of 17 10/23/2010 7:43 PM
Next turn off all the layers so all you see is the image and yourpoints. We will now begin to georeference the image. Using the Movetool in the georeferencing toolbar move the image so it lines up withyour points. It may be a bit off as far as zoom is concerned but thatsok for now. Just orient the image so it is square with the points.
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12 of 17 10/23/2010 7:43 PM
Using the Add Control Point tool start to match your image to yourpoints. First you click on the aerial image where your pointoriginated in Google Earth (In this example I will be using a railroadoverpass that I spotted earlier as a landmark). Drag your mousefrom there to the point in ArcGIS that corresponds to it. Your imagewill shift and the point should be on top of your image in the correctplace.
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13 of 17 10/23/2010 7:43 PM
Repeat this process for each point on your aerial until all points arematched up. You may have a small amount of residual, just playaround with it until you feel comfortable with the outcome.
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14 of 17 10/23/2010 7:43 PM
Once you have finished georeferencing the image click on theGeoreferencing toolbar and click "Update Georeferencing". Next click"Delete Control Points". At this point your crosshairs will disappear.Your image is now georeferenced. If you look in the folder whereyour image is stored you will see several new files that are used todisplay the image in ArcMap.
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15 of 17 10/23/2010 7:43 PM
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